When material processing is carried out with laser light, a laser power of only about several watts may be sufficient depending on the material to be processed. However, in a case where for example thick metal is to be processed, a laser power on the order of kilowatts is necessary. Examples of a laser source for metal processing include a YAG laser, a CO2 laser, an excimer laser and a fiber laser. Out of these, a fiber laser is a high-efficient, high-gain laser that produces good-quality laser light. Since the fiber laser is mainly constituted by optical fibers, the fiber laser is attracting attention as a laser source that is easy to maintain.
A fiber laser also has the following advantage. A path through which laser light propagates is also made of optical fibers, and for example the length of a delivery fiber that connects an output end part through which the laser light is outputted and a main body of the fiber laser is freely changeable. Therefore, it is possible to carry out material processing by moving only the output end part without moving the main body.
As is clear from above, a delivery fiber needs to be movable freely to some extent. Therefore, the delivery fiber may be more prone to breaks than the main body. If a break occurs in the delivery fiber, high-power light may be emitted from a site of the break and pass through a hose etc. that protects the delivery fiber. If this is the case, the light may escape into a surrounding area.
In view of this, for safety reasons, it is necessary to detect a break in the delivery fiber. Further, the stability of the power of laser light to be emitted is important for the purpose of achieving stable processing. Therefore, it is necessary to stabilize, by use of a light intensity monitoring circuit, the intensity of laser light to be outputted.
Conventionally, there has been a light monitoring device as shown in Patent Literature 1. The light monitoring device is a device in which (i) a slanted Bragg grating is provided in a core of an optical fiber so that light guided through the core is partly emitted to the outside of the optical fiber and (ii) a photoreceptor detects the light thus emitted. Further, Patent Literature 2 discloses a method of detecting scattered light leaking from a fused part of a fiber by another fiber near the fused part.